1. Field of the Invention
The present invention relates to a novel process for producing esters of monocarboxylic acids from vegetable or-animal oils.
The principal reaction is transesterification occurring in accordance with scheme I below, and possibly a coupled esterification and transesterification reaction, esterification occurring in accordance with scheme II below. In these schemes, the fatty acid chains are represented by oleic type chains.


Esters of fatty substances are currently used in a number of applications such as diesel fuel, domestic fuel, solvents, base compounds for the production of sulphonates of fatty alcohols, amides, ester dimers, etc.
When producing an ester from an oil and a monoalcohol, depending on the nature of the starting oil used, 10% to 15% of a secondary product, namely glycerin, is automatically formed. This glycerin fetches a high price and is sold for a variety of uses, but only if it is of high purity. This is achieved after profound purification steps in specialized vacuum distillation units.
When producing methyl esters from fatty substances starting from refined oils and alcohol, although simple alkaline derivatives, such as sodium alcoholates, sodium hydroxide or potassium hydroxide, are currently used as catalysts under fairly mild conditions (temperature of 50° C. to 80° C. and atmospheric pressure), as can be seen from a number of patents or publications such as JAOCS 61, 343-348 (1984), a pure product that can be used as a fuel and a glycerin that satisfy specifications are only produced after a great many steps.
If, for example, the most frequently used catalysts are taken, both the glycerin and the ester contain those alkaline compounds, which must be eliminated by washing and/or neutralization in the ester fraction, then drying. In the glycerin phase, the soaps and alcoholates present must be neutralized, and salts, which are sometimes formed, have to be eliminated.
The glycerin obtained generally contains 5% to 40% by weight of water. It also contains salts from neutralizing the alkaline catalyst, for example sodium chloride when the catalyst is sodium hydroxide or sodium methylate and when neutralization is carried out with hydrochloric acid. The concentration of salts in the glycerin from such processes is generally in the range 3% to 6% by weight. The production of high purity glycerol from glycerin from such processes thus involves purification steps such as reduced pressure distillation, which can sometimes be combined with exchange resin treatments.
In summary, the majority of commercial processes for producing esters can relatively easily produce heavy products (esters and glycerin), which must be purified a great deal using a variety of treatments, which in the end affect the cost of transformation.
It has now, surprisingly, been discovered that it is possible to obtain esters of said monoalcohols and a glycerin that is free of salts, in 1 to 3 steps, under particular conditions, directly from vegetable or animal oils and monoalcohols, and in any case not containing more than 5 ppm, with a purity in the range 95% to 99.9%, usually in the range 98% to 99.9%, by using as the catalyst a particular heterogeneous catalytic system, either continuously, for example in a fixed bed, or discontinuously.
2. Description of the Prior Art
The use of heterogeneous catalysts is not novel.
Examples of prior art documents dealing with heterogeneous catalysts that can be cited include European patent EP-B-0 198 243. The transesterification catalyst, which transforms oil and methanol into the methyl ester, is an alumina or a mixture of alumina and iron oxide. In the examples, the column used for the fixed bed has a volume of 10 liters and in general, oil is injected at a flow rate of less than 1 liter/hour, which produces an HSV (HSV=hourly space velocity=volume of oil injected/volume of catalyst/hour) of less than 0.1. For a factory producing 100,000 tons/yr., this would correspond to reactors of at least 150 m3.
A further problem that appears to arise is that of the quantity of glycerin recovered, which is much lower than theory predicts. None of the examples that claim to collect 10% of glycerin even approaches that value. Finally, the purity of the esters is quite low, 93.5% to 98%. What becomes of the glycerin that is not recovered is not stated. In certain cases, glycerin ethers are formed, as indicated in the patent; in other cases, it may perhaps decompose, unless it is eliminated in a first step. Thus, the performance level is fairly low. It should be indicated that at the indicated HSVs and for a contact time of more than 6 hours, a conversion of 80% and more can be obtained even without a catalyst.
Thus, that patent does not appear to provide a reasonable solution from the point of view of economics.
Other references exist in the literature, this time mentioning zinc oxide, but in reactions for the esterification of glycerin with a fatty acid [Osman in “Fette Seifen und Anstrichmittel”, 331-33 (1968)]. In that work, about twenty catalysts are compared at 180° C. in a discontinuous process. There is practically no difference between zinc chloride, zinc sulfate, zinc powder, barium oxide, calcium oxide, zinc oxide, alumina, thiosalicylic acid, calcium phosphate, potassium bicarbonate, sodium methylate or ethylate and even lithium hydroxide. All of the salts or oxides yield between 32% and 39% of monoglyceride in a comparative test in which an excess of glycerin is used with respect to the fatty acid.
U.S. Pat. No. 5,908,946 describes a process which can function continuously or discontinuously using solid non-soluble catalysts. However, the catalysts used are either zinc oxide or a mixture of zinc oxide and alumina, or a zinc aluminate.